Simple R.F. Meter
available test equipment for hobbyists in the 21st century is pretty
high quality at a relatively low price, so the motivation for buying
parts and building your own RF power meter has to be driven not by cost,
but by a desire to gain the experience. This article presents a simple
RF power meter that can be built for about $25 worth of parts. All the
parts should be readily available except for the Raytheon CK-721 (or
CK-722) germanium PNP transistor. The
2N3906 and the
2N2907 have been suggested as replacements, but some changes in
the biasing resistors might be required.
October 1958 Popular Electronics
[Table of Contents]People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights (if any) are hereby acknowledged.
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Simple R.F. Meter
By Charles J. Schauers
Measure r.f. output of the transmitter
in your ham shack ... or use this meter in -a dozen other ways
inexpensive r.f. indicator has a wide variety of uses around the ham
shack or mobile radio installation. It can act as an absorption frequency
meter (if calibrated), a field strength meter, neutralization indicator,
or modulation monitor with phones. However, the main job of the model
shown is to indicate proper antenna loading for my "minified" mobile
It is relatively simple to put together, and nearly
any low-priced transistor will work well. However, for maximum sensitivity,
a transistor with a beta (current amplification) of .between 25 and
45 should be used.
Construction. A 3 3/4" x
3" x 2 1/8" aluminum chassis houses all parts. The coil (L1) is tapped
and connected. to switch S1 before installation in the box to facilitate
soldering. Then the coil is cemented to the box by its plastic support.
Care should be taken in soldering the crystal diode (CR1) into
the circuit by "heat sinking" the connections with a pair of long-nose
pliers. A socket should be used for the transistor.
1 1/2-volt cell to the chassis with household cement. With normal use,
it should last almost its shelf-life.
After the unit is turned
on, zero the meter with potentiometer R3 in the collector circuit. Attach
a small wire to the input binding post on the rear of the box which
feeds r.f. to the tuned circuit, and you are in business.
Applications. If the device is to be used as an absorption
frequency meter, it can be calibrated with a heterodyne frequency meter
coupled to the input post through a small (about 500-μμfd.) capacitor.
The simplicity of the r.f. meter circuit (see
schematic below) makes for ease of layout on the chassis (above). Number
of turns tapped on L1 for each band is indicated on the schematic. Finished
meter is shown at top.
A 2-1/2" length of wire is sufficient
for r.f. pickup when checking oscillator, doubler or buffer and final
amplifier stages of a transmitter.
When using the indicator
as a field strength meter to adjust a beam antenna, the pickup wire
length will depend upon the distance from the antenna and how much power
is being applied from the transmitter's final amplifier. Usually, a
2" piece of wire will afford sufficient pickup at 100 feet with the
average low-power transmitter when the device is hand-held.
To provide some attenuation of very strong signals, the indicator can
be used harmonically. Set the bandswitch to 40 meters when you want
to measure carrier strength on 80 meters.
As a means for tuning
mobile or fixed transmitters (especially those employing pi-output-networks),
this unit enables one to determine very quickly if the antenna and not
the pi-network is taking the load. For mobile operation, the regular
auto broadcast antenna can be used for signal pickup. However, the device
should be harmonically operated as described above because of the strong
signal present. If the auto antenna is not used, try a small piece of
insulated wire taped to the inside of the front bumper, connected with
shielded wire to the indicator.
If you are interested in monitoring
your modulation, a pair of magnetic phones can be connected in the collector
circuit of the transistor. The meter is disconnected (as well as the
top of the balancing potentiometer) and the phones are connected between
battery minus and collector.
B1 - 1.5·volt D cell
C1 - 140·μμfd. miniature
C2 - 0.002·μμfd. mica capacitor
CR1 - Crystal
diode (Sylvania 1N34 or equivalent)
L1 - #24·wire, 1"·diameter coil
(Airdux 832T or B & W 3016 - 32 turns per inch, tapped as shown
M1 - 0-1 ma. d.c. meter
R1, R2 - 680·ohm, 1/2·watt
R3 - 6500-10,000 ohm slotted shaft potentiometer
- 1-p., 4-pos. rotary switch
S2 - S.p.s.t. toggle switch
- Transistor (General Transistor GT·87 or GT-88
or Raytheon CK·721)
1-Aluminum chassis box (LMB·135)
HOW IT WORKS
Radio frequency energy tuned by
L1-C1 is applied to diode CR1. The rectified current then takes a path
through the base-emitter circuit of transistor TR1.
amplification occurs and is read by the 0.1 milliammeter. Capacitor
C2, connected between the transistor base and ground. bypasses the radio
frequency. The greater the strength of the r.f. signal picked up, the
higher the reading on the meter.